The present invention provides a finned tubing for heat exchangers having improved heat transfer characteristics for heating or cooling.
In one form of heat exchange, a gas is caused to flow past and in contact with the exterior of tubing which contains a liquid or a gas of a different temperature flowing therethrough. An example of such heat exchange is provided by a typical Air Cooled Heat Exchanger employed for fluid cooling or condensing. The fluid to be cooled or condensed passes through parallel disposed and manifold connected finned tubes which are generally arranged in layers. Ambient air is forced transversely across the finned tubes by either forced or induced draft fans. Fins are conventionally provided on the tubes to increase the effective heat transfer surface. Generally, the heat-transfer coefficient of the fluid inside the tubes is greater than the heat transfer coefficient on the outside finned surfaces which are in contact with the moving air.
Smooth fins provide the least resistance to air flow and consequently require the least fan horsepower to move the air past and in contact with the fins. However, because the fins were smooth, they did not create sufficient turbulence for optimum heat transfer. Roughness on fin surfaces, protrusions, or edges that break up the air flow over the fins resulted in added turbulence which improved the rate of heat transfer per unit of external fin tube surface. However, the more the air flow is broken up to add turbulence, the more horsepower is needed to force the air between the fins and through the rows of finned tubes.
In the past helically wound fins have been cut, notched, serrated, and twisted to provide tortuous paths for the air to add turbulence. However, the structure to create the turbulence has often also provided excessive resistance to air flow and thus has been counterproductive, as the overall energy balance has been made unfavorable by requiring excessive power to move the air across the heat transfer surfaces at minimal additional gain in heat transfer.
In addition, in the past, serrated tubing has been formed by completely or partially cutting the serrations by cutting tools. For example, in one method, the fins are formed on the tube, and the serrations thereafter cut therein. In still another method, one portion of each serration is pre-cut, with the remainder of the serration tearing as the fin is formed on the tube. Such prior art methods were either time consuming, difficult to control, costly, or required specialized equipment.